U.S. patent number 4,978,437 [Application Number 07/009,075] was granted by the patent office on 1990-12-18 for method of applying optical coatings of silicon compounds by cathode sputtering, and a sputtering cathode for the practice of the method.
This patent grant is currently assigned to Leybold Aktiengesellschaft. Invention is credited to Peter Wirz.
United States Patent |
4,978,437 |
Wirz |
December 18, 1990 |
Method of applying optical coatings of silicon compounds by cathode
sputtering, and a sputtering cathode for the practice of the
method
Abstract
Method of applying optical coatings of silicon compounds to
substrates by reactive cathode sputtering of siliceous target
materials. To solve the problem of improving the utilization of the
target material and eliminating the blowout of particles from the
target, the target of the invention is a polycrystalline silicon
casting of at least 99% silicon containing dopants from the group,
boron, antimony, phosphorus and arseic, and it is sputtered by
direct current in an atomsphere containing the reaction gas.
Inventors: |
Wirz; Peter (Waldernbach,
DE) |
Assignee: |
Leybold Aktiengesellschaft
(Hanau I, DE)
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Family
ID: |
6235715 |
Appl.
No.: |
07/009,075 |
Filed: |
January 27, 1987 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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729294 |
May 1, 1985 |
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Foreign Application Priority Data
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May 12, 1984 [DE] |
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3417732 |
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Current U.S.
Class: |
204/192.23;
204/192.15; 204/192.26; 204/298.13 |
Current CPC
Class: |
C23C
14/3407 (20130101); C23C 14/3414 (20130101) |
Current International
Class: |
C23C
14/34 (20060101); C23C 014/34 () |
Field of
Search: |
;204/192.1,192.12,192.15,192.18,192.22,192.23,192.26,192.27,291 |
References Cited
[Referenced By]
U.S. Patent Documents
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4251289 |
February 1981 |
Moustakas et al. |
4365015 |
December 1982 |
Kitajima et al. |
4374391 |
February 1983 |
Camlibel et al. |
4380557 |
April 1983 |
Ishioka et al. |
4430185 |
February 1984 |
Shimomoto et al. |
4508609 |
April 1985 |
Moustakas et al. |
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Foreign Patent Documents
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2461763 |
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Mar 1981 |
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FR |
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0109325 |
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Jul 1982 |
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JP |
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Other References
Suzuki et al., "Doping Effects of Group III and V Element on a --Si
Prepared by High Pressure rf Sputtering", Japanese Journal of
Applied Physics, vol. 19 (1980), Supplement 19-2, pp. 85-89. .
Thorton et al., "Internal Stresses in Amorphous Silicon Films
Deposited by Cylindrical Mag. Sputtering Using Me, Ar, Kr, Xe and
Ar+H.sub.2 ", J. Vac. Science Technol., 18(2), 3/81, pp.
203-207..
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Primary Examiner: Nguyen; Nam X.
Attorney, Agent or Firm: Felfe & Lynch
Parent Case Text
This application is a continuation, of application Ser. No.
729,294, filed May 1, 1985.
Claims
I claim:
1. Method for applying optical coatings of compounds of silicon to
substrates by reactive cathode sputtering of siliceous target
materials, comprising: sputtering as the target material a cast and
solidified-from-the-molten-state polycrystalline silicon formed
body of at least 99% silicon with at least one dopant selected from
the group consisting of boron, antimony, phosphorus and arsenic,
said silicon formed body containing said at least one dopant only
in the range such that said silicon formed body has a specific
resistance of 0.5 to 10 ohms.times.centimeter by direct current in
an atmosphere containing the reaction gas.
2. Sputtering cathode as a target for applying an optical coating
to a substrate comprising: a formed body of siliceous material as a
target for applying an optical coating to a substrate, the target
comprising a cast and solidified-from-the-molten-state
polycrystalline silicon formed body of at least 99% silicon with at
least one dopant selected from the group consisting of boron,
antimony, phosphorus and arsenic, said silicon formed body
containing said at least one dopant only in the range such that
said silicon formed body has a specific resistance of 0.5 to 10
ohms.times.centimeter.
Description
BACKGROUND OF THE INVENTION
The invention relates to a method of applying optical coatings of
silicon compounds to substrates by reactive cathode sputtering of
siliceous target materials.
It is known to use, for the production of coatings of silicon
compounds, a planar target material which has been formed under
high pressure from silicon powder containing additional materials
such as aluminum and lithium to produce electrical conductivity and
improve thermal conductivity. For the achievement of a high density
of the target material, the compression is performed at high
temperatures on the order of 2000.degree. C. Due to this complex
manufacturing process such targets are very expensive. For example,
a target plate measuring 70 cm.times.25 cm.times.1 cm costs between
DM 25,000.00 and DM 40,000.00. At the same time, hot-pressed target
material has the disadvantage that, due to its still-grainy or
porous texture, it has a large surface area, and this necessitates
a relative long period of time for the process of cleaning the
target material for each new batch. This means a preliminary
sputtering of the target surface onto a plate or diaphragm
temporarily inserted between the target and substrate. The
correspondingly long time required for the cleaning process results
in a corresponding loss of material, but also makes the sputtering
apparatus less economical to operate, since the actual coating
process does not begin until the cleaning process is completed.
Moreover, the surface of a pressed material is rough, from the
microscopic viewpoint, and as the sputtering process advances
so-called bumps or humps can form on the surface. This is to be
attributed basically to the different rates of sputtering of the
components contained in the target material, such as aluminum and
lithium, for example. On account of the low thermal conductivity
between the humps and the rest of the material a local overheating
occurs which causes solid particles of the target material to burst
out of the target plate. Even if these particles do not come in
contact with the substrates, however, this process has a negative
effect on the growth and homogeneity of the layer deposited on the
substrate.
Reactive cathode sputtering is chiefly a process of oxidation that
occurs while the coating is in progress, i.e., oxygen is contained
in the reaction gas, and oxides of silicon, mostly silicon dioxide,
are formed. Similar considerations apply, of course, to other
reaction gases, nitrogen for example, if silicon nitride is to be
formed on the substrates.
The above-described reactive processes are not free, either, of
adverse effects on the target materials. In the case of hot-pressed
target materials, there is especially the tendency to form on the
sputtering surface layers of the product of the reaction with the
reaction gas, that is, layers of silicon dioxide in the case of
oxygen. Such reaction products are electrical nonconductors, so
that the sputtering process is at least locally hampered, or in any
case is impeded unless a high frequency is used for the sputtering
process. On account of the greater simplicity of the technique,
however, there is a tendency to use so-called diode sputtering
insofar as possible, along with a magnetic field enhancement of the
sputtering process in some cases, in order to raise the otherwise
low sputtering rate by at least one power of ten. The classical
hot-pressed target material, however, leads to the problems stated
above.
The article by Thornton and Hoffmann, "Internal Stresses in
Amorphous Silicon Films Deposited by Cylindrical Magnetron
Sputtering using Ne, Ar, Kr, Xe and Ar +H.sub.2 ", which appeared
in J. Vac. Sci. Technol., 18 (2), Mar. 1981, pages 203 to 207,
discloses the use of polycrystalline silicon, drawn in rod form
from fusion, as a cathode in a sputtering process wherein the
cathode is provided with doping to achieve the necessary electrical
conductivity, and the doping material has to be diffused into the
cathode material by an exceedingly tedious process. The described
cathode sputtering process is not part of a production process, but
serves only for the study of internal tensions in the coatings thus
produced. The production of the rod-shaped cathode calls for the
insertion of a narrow slit diaphragm, so that a very large part of
the material sputtered away from the cathode is deposited on the
diaphragm, not on the substrate. In any event, this state of the
art has to do with the production of coatings having special
electrical properties, not coatings for optical applications.
The article by Severin, entitled, "Materialien fuer die
Kathoden-Zerstaeubung," which appeared in Vakuum-Technik, vol. 33,
No. 1, pp. 3 to 9, discloses the production of cast targets made
from an aluminum-silicon alloy. The aluminum content, however, is
very high, since the aluminum is intended to produce not only a
good electrical conductivity permitting sputtering by direct
current, but also to permit plastic mechanical deformation for the
purpose of subsequent "grain fining." With such targets, however,
homogeneous coatings of virtually pure silicon dioxide cannot be
produced, since at least aluminum oxide is an important component
of the coatings, and silicon and aluminum behave differently in the
sputtering, so that the relative proportions of the oxides change
as the consumption of the target progresses.
Optical coatings of silicon compounds, especially those of silicon
dioxide, however, are playing a vital role ino today's coating
technology, both as a final protective coating on large-area
substrates such as windowpanes, for example, and as a material for
multiple coatings built us of alternating high and low refraction
layers (known as "interference filters").
It is therefore the object of the invention to device a method of
the kind described above, in which the utilization of the target
material, consisting of at least 99% silicon, will be improved and
the danger of particles bursting out of the target material will be
eliminated or at least greatly reduced.
BRIEF DESCRIPTION OF THE DRAWINGS
The single FIGURE is a diagrammatic view of the apparatus partly in
section for practicing of the method of the invention.
THE INVENTION
In the method described above, this object is achieved in
accordance with the invention in that a polycrystalline silicon
casting, solidified from fusion and composed of at least 99%
silicon plus dopants from the group, boron, antimony, phosphorus
and arsenic, is sputtered by direct current in an atmosphere
containing the reaction gas.
In the method of the invention, therefore, the casting, which is as
a rule a plate defined by plane-parallel surfaces, is produced from
molten material by a casting process in which polycrystalline
silicon is formed. This is virtually pure silicon, which contains
only dopants in the parts-per-million range, and is not
mechanically deformable. In a casting process of this kind, the
dopants can be distributed uniformly in the melt by simple
admixture, so that a time-consuming diffusion process is
unnecesary. The dopants serve only to produce sufficient
conductivity during the sputtering process, not for the production
of a defined conductivity in the deposited coatings, which in any
case is only an accompanying phenomenon of secondary
importance.
If the target is of parallelepipedal shape, it can easily be
fastened on a cathode base which as a rule is intensively cooled by
a water circuit. Thus the target plate, too, can be intensively
cooled. Other advantages of plate-like targets are low losses of
material and high efficiency in the coating of planar
substrates.
In the sputtering process, it is found, first of all, that even on
the microscopic scale, a smooth surface is preserved on the target,
which is far less inclined to oxidize than sintered or pressed and
thus porous target plates. Such a plate also has adequate and,
above all, uniform thermal conducting properties, so that local
overheating no longer possible. Consequently, the electrical power
per unit of area of the target material can be considerably
increased, thus also increasing the sputtering range. And yet no
blowout of surface particles from the target has been observed.
Since the sputtering process is thus more uniform, greater
homogeneity has been maintained within the coatings produced. The
costs of a target plate in accordance with the invention, having
the dimensions given above, are only a fraction of the cost of a
corresponding hot-pressed target plate.
Above all, however, the deposited coatings contain virtually no
more dopants, since they have been reduced in the sputtering
process by pumping down or evacuation, and cannot reach the
substrates. For this reason the indices of refraction of the
produced coatings are identical to those which are produced by the
reactive, high-frequency sputtering of undoped material.
The invention also relates to a sputtering cathode for the practice
of the process of the invention in conjunction with a so-called
magnetron cathode, in which the plasma discharge is defined by a
closed tunnel of magnetic lines of force to a narrow space in the
immediate vicinity of the target surface.
Such a target consists, for the solving of the same problem, of a
polycrystalline silicon casting solidified from the molten state
and composed of at least 99% silicon with dopants from the group,
boron, antimony, phosphorus and arsenic.
DESCRIPTION OF A PREFERRED EMBODIMENT
An embodiment of an apparatus for the practice of the method of the
invention will now be explained in conjunction with the drawing,
which is a vertical cross section taken through a complete
apparatus.
In the drawing there can be seen a cathode system 1 with a
supporting plate 2 which is fastened through an insulator 9 to the
roof of a vacuum chamber 10. All of the parts of the cathode system
which are essential to its operation are fastened also to the
supporting plate 2. This includes a trough-like hollow body 3 which
consists of a nonmagnetic material such as copper, for example, and
is removably attached to the supporting plate 2. The hollow body 3
has a bottom 4 which is parallel to the supporting plate, and on
which is mounted a consumable target plate 5 of the silicon cast in
accordance with the invention. This hollow body can be attached in
a conventional manner, e.g., by soldering.
Inside of the hollow body 3 there is a magnet system 6 which
consists of a magnet holding plate 7 parallel with the supporting
plate 2, and a group of permanent magnets 8 which are fastened to
the magnet holding plate 7. The magnets 8 are arrayed on the back
of the target plate 5 with alternating polarity. Due to the
polarity and to the relative position of the magnets with respect
to the target plate, the lines of force assume the pattern
indicated by broken lines, i.e., the lines of force emerging from
one pole pass through the target plate and, on an arcuate course,
re-enter the target plate in the area of the other pole. This
pattern of the magnetic lines of force produces a so-called
magnetic tunnel, which makes it possible to increase the sputtering
rate substantially, but at the same time causes the target material
to be ablated more intensely in the area of the tunnel, and most
intensely in the middle of each tunnel.
Between the magnet holding plate 7 on the one hand and the
supporting plate 2 on the other there is disposed an adjusting
means 11, not explained in detail, which can be operated by an
adjusting knob 12, and by which the distance between the magnet
system 6 and the target plate can be varied, in order for example
to compensate for the effect of the sputtering or erosion pits
which become increasingly deep during the sputtering process.
The cathode system 1 is connected to a direct-current voltage
source, also not shown, which supplies a suitable sputtering
voltage between, say, 200 and 800 volts. Also not shown is the
grounded shield which surrounds the side walls of the cathode
system 1 and which prevents the formation of glow discharges at
these undesired places. Reference is made to German publication OS
30 47 113 for further details which, with the exception of the
target material, pertain to the state of the art.
In the vacuum chamber 10, a substrate holder 13 is disposed
opposite the target plate 5, and the substrates 14 which are to be
coated are mounted thereon. The vacuum chamber 10 is connected by a
vacuum line 15, shown in part, to a set of vacuum pumps whereby a
vacuum suitable for the sputtering process can be produced, of
between 5.times.10.sup.-2 and 10.sup.-4. The reaction gas needed
for the sputtering process is delivered by a conduit 16 provided
with a metering valve 17.
EXAMPLE
In an apparatus in accordance with the drawing, substrates of
optical glass measuring 20 cm.times.20 cm.times.0.5 cm are arranged
on the substrate holder 13. The distance between the top of the
substrates and the face of the target plate was 20 cm. The target
plate was composed of "tiles" of a size of 10 cm.times.10 cm and
had surface dimensions of 80 cm.times.20 cm and a thickness of 10
mm. The target material was silicon doped with boron and having a
specific resistance of about 3 ohms centimeter, prepared by
polycrystalline solidification from the molten state in a mold.
After the customary preliminary evacuation, the surface of the
target plate was cleaned for 5 minutes against a plate inserted
over the substrates 14. At the end of the cleaning process, a
pressure of 10.sup.-2 mbar was established by the continuous
admission of a mixture of argon and oxygen in a ratio of 10:1. In
the sputtering process then performed for a period of 3 minutes, a
coating of silicon dioxide 420 nm thick was deposited on the
substrates. At a sputtering voltage of 585 volts and a current to
the ground of 35 amperes, a specific sputtering power of 12.8
W/cm.sup.2 is computed with respect to the free surface area of the
target plate 5. The specific sputtering power can be considered to
be relatively very high (in comparison to about 6 W/cm.sup.2 in the
case of a hot-pressed target plate).
Nevertheless, no blowout of particles from the target plate could
be observed. The coating thickness deviations amounted to less than
3%.
The percentages stated with respect to the silicon content of the
target are percentages by weight.
* * * * *